The present invention relates to the sector of transmitting and receiving microwave signals, and more precisely to a subharmonic frequency converter for applications to the millimetric radio waves.
In the range of the millimetric radio waves concerning frequencies from 30 GHz upwards it is possible to realize very compact and miniaturized frequency converters, thanks also to the reduced dimensions of the cross sections of the rectangular waveguides interconnecting with those.
This type of converters are generally realized in thin film, and the layers are put into metal packages. The frequency conversion is carried out by mixer diodes. A problem coming up regarding the propagation of the radiofrequency signal from the waveguide to the diodes, and vice versa, is that of carrying out a transition between the metallic waveguide and the circuit in microstrip connected to the diodes.
A first known solution for this problem consists in the introduction into the guide of an end of a small cylindric conductor which other end is welded to the microstrip, as for example disclosed in the U.S. Pat. No. 4,679,249 patent document. In the frequency converter of the citation the above-mentioned conductor operates as an antenna in the guide and transfers the here present radiofrequency signal to the microstrip, or vice versa.
The U.S. Pat. No. 4,955,079 patent document discloses a waveguide excited subharmonic mixer with image rejection realized in thin film technology. As known a subharmonic converter is generally based on a couple of mixer diodes connected each other in antiparallel and forming a single-ended structure unbalanced to ground as regards both the RF and local oscillator signals.
The original embodiment of the second mixer utilizes the dielectric substrate of the thin film circuit for mechanically supporting the RF waveguide, providing in the meanwhile means for including the pair of diodes inside the cavity of the waveguide. Because the radiofrequency signal directly illuminates the mixer diodes inside the waveguide, the additional guide to microstrip transition means employed in the converter of the first citation is now avoided. For the aims of above the dielectric substrate includes a rectangular groove extending through all the thickness of the substrate and partially encircle the pair of diodes. The end rim of the radiofrequency waveguide is inserted in the groove and firmly grounded on the backside of the substrate. The RF rectangular waveguide is mounted on the front side of the dielectric substrate (slab) by forcing against the substrate a peripheral metallic flange belonging to the same waveguide. A second short tract of RF rectangular waveguide is mounted on the back side of the slab opposite the former waveguide for the purpose of furnishing a proper termination for the RF signal beyond the diodes. The back waveguide tract has a second metallic flange forced against both the ground plane and the rim of the former waveguide out of the grooves, by screwing each other the two flanges. A portion of the ground plane is without metallization in correspondence of a rectangular area inside the back waveguide tract. A very small line is provided which departs from one end of a pair of diodes and prosecutes through the internal side of the rectangular groove towards the ground plane on the backside.
It is all evident from the above description the complexity of the means provided for coupling the RF waveguide to the pairs of unbalanced diodes and the weakness of the resulting structure. In particular the rectangular groove and the further four holes necessary to secure the rectangular flanges increase the risk of breaking the substrate in the included area. Besides a metallic RF waveguide anchored to a rigid and breakable dielectric substrate, it doesn""t resemble a good design. In fact if employing the converter in particular hazardous environments such as military or on board satellite, the strong mechanical vibrations that the elongate RF waveguide communicates to the dielectric substrate can easily break the latter.
Another drawback is that the technical solution for grounding the diodes doesn""t appear sufficiently reliable because of the difficulty to perform a so small and particular metallization which could easily undergo breaking or detach during the operating life of the converter.
Furthermore from the thin film circuit a spurious radiation can propagate in the free space around and disturb the other circuits of the communication apparatus including the mixer. The vice versa is also true.
In the end the groove around the diodes for the rim insertion of the front RF waveguide requires the use of expensive drilling means, such as an ultrasonic drill or a C02 laser. This type of drilling is a burden operation in a large scale production.
Accordingly the purpose of the present invention is to overcome the above drawbacks and indicate a microwave subharmonic frequency converter in single-ended configuration having such a good mechanical stability, reliability, and protection against spurious radiation to render it particularly useful for the employment in environments characterized both by strong vibrations and low noise requirements, such as on board satellite.
To achieve said purposes the object of the present invention is a microwave subharmonic frequency converter with the features of the claim 1.
In particular, the mechanical stability and protection against spurious radiation are both reached by lodging and securing the dielectric substrate supporting the thin film converter thoroughly inside cavities of a metal body closed with a metal plate. Among these cavities a first one, which includes a diplexer and a low pass filters, is not to be considered as an effective electromagnetic resonating cavity but more properly a cavity for reaching the purposes stated above, while a second one including the diodes is a true RF resonating cavity which extends to a RF waveguide fixed to the metal body. A third small additional cavity extends the first cavity beyond the second one for the only purpose of grounding the diodes without directly contacting the internal walls of the RF resonating cavity, in that preventing a possible anomalous propagation of the RF signal and the level increasing of the spurious propagation mode.